The objective of this project is to gain some understanding of the role of two early functions of Polyoma in transformation: the hr-t function and the ts-a function. Complementation between the two classes of transformation defective mutants representing the two genes has been shown to lead to stable transformation, indicating that the two functions govern different steps of transformation. This project deals with the analysis of the properties of the transformed clones obtained by complementation referred to as complementation clones. The particular interest lies in asking which viral function(s) are required to persist in complementation clones, and what is their relative topology in the host cell. The method involves the analysis of rescued virus, of the viral integration and of the proteins produced in the transformed cells.